This invention relates in general to dispersion processes and, more specifically, to a process for forming dispersions of particles in a solution of a film forming binder.
A photoconductive layer for use in electrophotography may be a single layer or it may be a composite layer. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between the contiguous charge transport layer and the supporting conductive layer. In another embodiment, the charge transport layer is sandwiched between the supporting electrode and a photoconductive layer.
Photosensitive members (photoreceptors) having at least two electrically operative layers as described above provide excellent images when charged with a uniform electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles. A key component of modern photoreceptors is the charge generation layer (CGL) which absorbs imaging light and produces the conducting charge that is used to discharge the charge on the photoreceptor surface and hence form an electrostatic image. The material component of the charge generation layer that performs this photogeneration of charge is a polycrystalline pigment. For reasons of electrical performance and cycling stability it is desirable that the charge generation layer be as thin as possible, yet absorb more than 90 percent of the light to which it is exposed. Thus, it is desirable to coat charge generation layer to a thickness of about 0.1 micrometer to about 0.2 micrometer (100 nanometers to 200 nanometers) taking into account the binder polymer. To coat such thin layers requires that the pigment particles be smaller than about 0.1 micrometer. (100 nanometers) Furthermore, the presence of larger pigment agglomerates causes a non-uniformity of photoelectrical response and is one of the causes of undesirable charge deficient spots (CDS). Hence, pigment dispersions require milling of the pigment in the presence of a binder polymer and solvent. Techniques such as shaking or rolling the dispersion with steel or other hard material ball (shot) are used to accomplish this milling.
The grinding of particles followed by dispersion of the ground particles in a solution requires prolonged grinding times and multiple processing steps. These grinding processes may require days of milling to achieve the proper particle size. Moreover, the mechanical grinding action of the shot damages the crystal surfaces and also leaves behind residue of the grinding (shot) material, e.g. metal particles in case of steel shot. Both effects can be detrimental to the electrical performance of the charge generating layer. The grinding can also involve, for example, pressure and shearing forces of attriters using considerable energy. Prolonging the grinding treatment in order to achieve the smallest possible particle sizes adversely affects the performance of high purity materials such as photoconductive or other materials such as pharmaceutical materials. Further, the longer the grinding treatment is continued, the worse the performance of the ground material product. For example, ground photoreceptor particles become less and less responsive and sensitive, the longer grinding continues. To obtain finely ground particles with conventional grinding processes, days of grinding are required to reduce materials such as metal-free phthalocyanine photoconductive particles into particles on the scale of tens of nanometers. This poses a practical difficulty, due to the time demand and also due to the detrimental effects of long milling described above. Thus, an optimum time, with an attendant trade-off in particle size and therefore charge generating layer thickness, is often chosen. However, even to achieve particle sizes on the scale 0.05-0.1 micrometers requires several hours of milling. Long grinding also consumes excessive energy and forms a large size distribution of particles, including undesirable agglomerates. Further, grinding medium residue, even in parts per million amounts, produce significant adverse effects in sensitive photoconductive materials such as dark decay, cycle up, residual voltage, and the like. These grinding medium residue include materials such as metals, metal salts, ceramic particles, and the like, which can also cause charge deficient spots in photoreceptors. Although some grinding media leave less residue, they tend to be more expensive. In addition, the material being pulverized tends to adhere to the grinding medium leading to loss of product when the grinding medium is separated from the material being pulverized. Prolonged grinding can also increase the damage to the crystal structure of some particles. Such damage to crystal structure can adversely affect the electrical performance of some photoreceptor materials.